One type of earth boring bit has cones that rotate as the bit revolves around a bit axis. Each cone has a number of rows of cutting elements. In one type, the cutting elements comprise tungsten carbide inserts pressed within holes formed in the exterior surface of the cones. Each insert has a cylindrical barrel that fits in an interference fit in one of the holes. A cutting end or tip is integrally formed with the barrel and protrudes from the hole. The cutting tip may be of a variety of configurations, such as hemispherical, ovoid, or chisel-shaped.
Typically, in the prior art, the holes are drilled in annular lands formed on the exterior of the cone. The land is normally cylindrical or conical. The depth of the hole sidewall normally equals the length of the barrel. The land is thus flush with the junction between the barrel and the cutting tip. The entire cutting tip protrudes from the land.
In some bit cones, part of the holes may be drilled into individual spaced apart circular counterbores, rather than an annular land. In those cases, the sidewall of the hole has a depth, measured from the base of the counterbore, that equals the length of the barrel. The junction of the barrel with the cutting end is flush with the base of the counterbore.
A number of years ago, reduced projection inserts were used in the nose and inner rows of some bits manufactured by the assignee of this invention, particularly for hard formations. In those bits, the junction between the barrel and the cutting end of the reduced projection inserts was recessed in the hole. The depth of the hole sidewall was approximately 0.060 inch greater than the length of the barrel. As a result, only a portion of the cutting end protruded past the land. The recession was done to reduce the risk of coring due to lost inserts in the nose area. To the inventors' knowledge, inserts with recessed cutting tips were not located in the outer or heel row or the adjacent row in those bits. The design of recessing the cutting tips of the inserts in the nose and inner rows of certain bits was discontinued several years ago by the assignee of this application.
The outer row inserts encounter the most severe conditions during drilling. These inserts are more subject to breakage due to high bending forces imposed on the inserts. Inserts are made less resistant to breakage by increasing the toughness, but this decreases the hardness. A decreased hardness generally results in the insert wearing faster in hard formations. Also, outer row inserts are made more resistant to breakage by having a shorter cutting tip, which reduces the moment arm on the insert. However, in the prior art, it has been generally considered that reduced insert projection reduces the rate of penetration. Also, the loss of inserts, even with a shorter cutting tip, still occurs.